Sputtering is performed in a process chamber that is a vacuum chamber. In order to perform sputtering, a mechanical rotary pump is first operated to form a rough vacuum of 1 Pa and thereafter a cryo pump described in Japanese Patent Laid-Open Publication No. Hei 5-321832 is operated to form a high vacuum of about 10−7 Pa. Then, a process gas such as Ar or N2 is introduced in order to perform sputtering. A surplus part of the process gas is condensed in the cryo pump with progress of the operation, thus lowering a performance of the cryo pump.
In other words, the cryo pump condenses the surplus part of the process gas in a conventional technique. The process gas gets between a pump chamber and a heat shield plate because of a structure of the cryo pump. In the process gas between the room-temperature cryo pump chamber and the heat shield plate, gas molecules transfer heat from a room temperature, thus raising a temperature of the heat shield plate and lowering a refrigerating capacity and a condensing performance.
An exemplary conventional technique using a horizontal refrigerator is described in detail with reference to FIG. 1.
A vacuum chamber 10 serving as a process chamber is connected to a coarse vacuum pump 12 that is a mechanical rotary pump, a cryo pump 20, and a process gas introduction port 14 and is formed to be airtight. Target 16 and wafer 18 are arranged inside the vacuum chamber 10 in order to perform a process such as sputtering. Sputtering is performed in the process chamber 10.
A manner of the process is now described.
(1) The coarse vacuum pump 12 is operated to coarsely draw a vacuum of 1 Pa.
If a vacuum level is not higher than a certain level, the amount of heat entering from a room temperature is large because of heat transfer by gas molecules. Therefore, the cryo pump 20 cannot perform refrigeration. Moreover, the cryo pump 20 does not work well because too many gas molecules (in particular, H2O) or the like adhere to the cryo pump 20. Thus, it is always necessary to draw a vacuum by using a mechanical pump. Furthermore, in order to achieve a high vacuum only by the mechanical rotary pump, a load applied to the pump is large because the pump should be rotated at a high speed, for example. From a viewpoint of reliability during a long operation, the long operation in a high vacuum state requires the cryo pump 20.
(2) Then, the cryo pump 20 is operated so as to form a high vacuum of about 10−7 Pa inside the process chamber 10.
The cryo pump 20 refrigerates a louver 26, a cryo panel (that is also called as a second-stage panel because it is connected to a second (refrigerating) stage 22) 28, and the like to a solidification temperature of gas molecules or less, thus causing condensation and solidification of gas molecules on those components or absorption of gas molecules because of cooling of activated carbon. In this manner, the cryo pump 20 forms a high vacuum. An operation of the horizontal refrigerator 30 forming the cryo pump 20 is suitable for a long high-vacuum operation with high reliability, because an applied load is lower than that applied to a mechanical pump.
(3) A process gas such as Ar or N2 is introduced from the process gas introduction port 14 in order to perform sputtering.
A two-stage GM (Gifford-McMahon type) refrigerator 30 is usually used in the cryo pump 20. A high-temperature first (refrigerating) stage 21 includes a heat shield plate 24 covering a second (refrigerating) stage 22. The heat shield plate 24 is provided for shielding radiated heat from a room temperature, suppresses entrance of heat to the second stage 22, and improves a refrigerating capability. A louver 26 or the like is provided at a top end of the heat shield plate 24, thereby forming an entrance for gas molecules. The louver 26 condenses gas molecules having a relatively higher solidification temperature (H2O in particular), for example, because it is cooled by the heat shield plate 24. Moreover, the second stage 22 is cooled to about 10 K. Thus, the second stage 22 condenses hydrogen, oxygen, nitrogen, and the like. The second stage 22 also cools activated carbon contained as absorbent in a cryo panel 28, thereby causing absorption of a gas into fine holes in the activated carbon.
However, in the above process, the process gas such as Ar or N2 enters in a shield chamber space 25 between the vacuum chamber 10 and the heat shield plate 24, as shown with Arrow A. Gas molecules in the entering process gas transfer heat from a room temperature to the heat shield plate 24, thus raising a temperature of the heat shield plate 24, and lowering the refrigerating capability and the condensing performance of the second stage 22.
Japanese Patent Laid-Open Publication No. Sho 60-228779 describes that, in order to prevent the gas from getting between the vacuum chamber and the heat shield plate, a rib or a flange is provided to make the space narrower or a heat insulating panel is provided to close the entrance for the gas.